Technical Field
The technical field may relate to lasers and more particularly to fiber laser oscillators.
Background Information
The design of fiber laser oscillators differ from solid state laser oscillators due to the losses associated with the long lengths of gain media. A gain medium of a fiber oscillator may be on the order of meters in length as opposed to solid state crystals that are commonly in the range of centimeters. With over a magnitude greater gain length in fiber oscillators, the intrinsic material losses and the energy dynamics of the rare earth dopant have prohibited the design of a fiber oscillator for maximum pump utilization. The fiber optic driven power losses are increased as one oscillates a frequency close to the border of the host material's spectral transparency. In silica-based fiber, this background power loss can become significant above a wavelength of 1.7 microns (1700 nanometers). In addition, three-level laser operation in a fiber can lead to significant ground-state absorption (e.g., erbium-doped fiber lasers emitting at 1532 nm or thulium-doped fiber lasers operating below 1940 nm), increasing laser threshold and leading to a loss in optical power in long lengths of fiber. These high losses inherently limit the efficiency of the laser, particularly in fiber lasers with long gain medium lengths on the order of several meters.
The output signal power of a laser is dependent on the available and absorbed pump power according to conservation of energy. Laser action is realized in a resonant cavity through absorption of pump power in a gain medium with subsequent conversion of the pump power to signal power emitted from the resonator output coupler. The total pump power absorbed by the laser and the ability of the medium (and associated rare earth ion dopant) to convert the pump power to signal power, all factors considered to be equal, are inexorably coupled. If one desires more signal power, without regard for lasing efficiency, the laser may be pumped at greater and greater levels.
The design of laser oscillators is generally predicated on maximum pump absorption to generate the greatest amount of power. This is achieved by increasing the length of the medium or the dopant concentration. The length of the fiber gain medium may be increased at the sacrifice of excess background and ground state absorption losses. Alteration of dopant concentration is a less available option in fiber optics due to coupling of fiber content with the optical properties of the fiber such as numerical aperture (NA), mode propagation, splice-ability, damage threshold and overall cost. The lasing efficiency of a solid state laser is tailored using mode matching and beam conditioning strategies. This is a design lever or option that is not available in fiber optics due to the nature of the confined waveguide.
The design parameters of a fiber laser from a solid state laser based upon similar output characteristics of the laser diverge due to the differences in performance characteristics described above. For instance, to improve fiber laser efficiency, one generally decreases the fiber length to minimize ground state absorption and background losses. Since the pump absorption is coupled with gain medium length, as one shortens the gain medium length, less power is absorbed, thus reducing the total output power of the laser. In contrast, the design of a fiber laser for maximum output power is achieved by increasing total fiber length for a given level of pump power. This generally results in decreased laser efficiency due to the fiber optic driven power losses. Most practical applications demand an optimized combination of highly efficient operation with the maximum power available from the laser. Thus, a need exists for a way to increase the efficiency of fiber lasers.
FIG. 1 generally shows a prior art laser system 1 which may comprise a fiber laser oscillator 2 which may comprise or be connected to an oscillator optical pump source which may include one or more optical pump sources 4. Pump source 4 may, for example, be in the form of a discharge lamp (arc lamp, flash lamp) or a pump diode. Oscillator 2 may include a high reflector or high reflector mirror 6 downstream of pump source 4, an oscillator or resonator length or piece of rare earth doped medium or optical fiber 8 downstream of mirror 6 and a partial reflector or partial reflector mirror 10 which is downstream of piece of fiber 8 and may serve as an oscillator output of oscillator 2. Mirror 6 may be a high reflector fiber Bragg grating, dielectric coating or other suitable high reflector known in the art. Mirror 10 may be a partial reflector fiber Bragg grating, dielectric coating or other suitable partial reflector known in the art. System 1 may further include a pump remover or cladding mode stripper 12 downstream of mirror 10.
The various components of system 1 and the laser systems discussed hereafter may be connected or spliced to or onto one another at respective splices, connections or connectors 14. Pump source 4 at a downstream end thereof may be connected or spliced to mirror 6 at an upstream end thereof at the given splice or connector 14. Mirror 6 at a downstream end thereof may be connected or spliced to fiber 8 at an upstream end thereof at the given splice or connector 14. Fiber piece 8 at a downstream end thereof may be connected or spliced to mirror 10 at an upstream end thereof at the given splice or connector 14. Mirror 10 at a downstream end thereof may be connected or spliced to pump remover 12 at an upstream end thereof at the given splice or connector 14.
As known in the art, pump source 4 may produce pump light which, as represented at Arrow A, exits pump source 4 and enters the optical cavity (a.k.a. resonant cavity or optical resonator) comprising high reflector 6, doped fiber 8 and partial reflector 10 to produce a laser which exits the output/reflector 10 of the optical cavity and oscillator 2 and enters pump remover/stripper 12, as shown at Arrow B. Unused or unabsorbed pump light, which was not absorbed in the gain medium or fiber 8 of the fiber laser oscillator, also moves downstream to exit the output/reflector 10 of the optical cavity/oscillator and enters pump remover/stripper 12, as shown at Arrow C, so that remover 12 removes or separates the unused pump light from the laser. The unused pump light is shown being removed, separated or exiting remover 12 at Arrow D. The laser continues downstream out of remover 12, as shown at Arrow E. The unused pump light thus represents pump energy which was not used in forming or enhancing the laser and is therefore wasted energy. Moreover, the laser produced does not benefit from the power which might have been attained if the unused pump energy could have been harnessed to enhance or amplify the laser.
By way of example, operating a thulium-doped fiber laser at 1900 nm is difficult due to bulk silica losses and ground-state absorption losses in the Tm-doped fiber. The fiber length required to achieve maximum output power from an oscillator may result in 20% unused pump power. Thus, for instance, for 100 watts (W) of pump power provided to the oscillator, 20 W is unused or not absorbed in the oscillator and thereby completely wasted.
FIG. 2 generally shows another prior art laser system 1A which may be or comprise a master oscillator power amplifier. System 1A is similar to system 1 in some ways and may include a fiber laser oscillator 2 as previously described, which thus may include high reflector mirror 6, doped fiber 8 and partial reflector mirror 10. System 1A may include pump remover or cladding mode stripper 12. System 1A may also include a pump combiner 16, one or more amplifier pump sources/diodes 4A and another rare earth doped medium or fiber 8A.
System 1A may operate in the same manner as discussed above with respect to system 1 to the extent that system 1A includes pump source 4, high reflector 6, fiber 8, partial reflector 10 and remover 12, as likewise shown by Arrows A-E. In addition, system 1A provides an amplifier which may include optical pump sources 4A, combiner 16 and downstream doped fiber 8A. As known in the art, the laser may exit pump remover 12 and move downstream to enter combiner 16 (Arrow E), and pump sources 4A may also produce pump light which moves downstream (Arrows F) into combiner 16. The laser and the pump light from pump sources 4A may exit combiner 16 and enter the second doped fiber 8A to create a more powerful laser H exiting fiber 8A compared to the laser shown at Arrow E. However, the unused pump light/energy removed (Arrow D) from the laser (Arrow E) by pump remover 12 is still wasted, as is the case with prior art laser system 1 previously discussed.